1. Field of the Invention
This invention relates to an alkali metal chloride electrolyzing cell and, more particularly, to an alkali metal chloride electrolyzing cell for producing alkali metal hydroxide at a low voltage.
2. Description of the Prior Art
As the process for obtaining alkali metal hydroxide by the electrolysis of an alkali metal chloride aqueous solution, a diaphragm process has recently been taking the place of a mercury process from the point of view of preventing environmental pollution.
With the diaphragm process, there have been proposed several processes of using an ion-exchange membrane as a diaphragm in place of asbestos for obtaining an alkali metal hydroxide with higher purity and higher concentration.
On the other hand, from the viewpoint of energy saving which has recently been attempted on a world-wide scale, it is desired in this technique to minimize the electrolytic voltage.
As the means for reducing the electrolytic voltage, there have so far been proposed various ones such as proper selection of material, composition, and shape of anode or cathode, selection of particular composition of ion-echange membrane to be used or the kind of ion-exchange groups, and the like.
These means have their effects to some extent, but most of them have a limit as to the concentration of the resulting alkali metal hydroxide, i.e., the concentration is at a not so high level, and, when the concentration exceeds the level, there results a rapid increase in electrolytic voltage or a decrease in current efficiency, or else, the phenomenon of electrolytic voltage reduction does not last or durability becomes poor. Thus, all of the conventional processes are not fully satisfactory from the industrial point of view.
It has recently been proposed to electrolyze an alkali metal chloride aqueous solution using an electrolytic cell wherein an anode or a cathode comprising a gas- and liquid-permeable porous layer is closely contacted with the surface of a cation-exchange membrane of a fluorinated polymer, thus obtaining an alkali metal hydroxide and chlorine (see Japanese Unexamined Patent Publication No. 112398/79). This process enables to minimize electric resistance of a solution to be electrolyzed and electric resistance of a hydrogen or chlorine gas to be generated, which has been considered unavoidable in this technique, thus being extremely excellent as a means to conduct electrolysis at a much lower voltage than in the conventional art.
In this process, the anode or cathode is bound to the surface of the ion-exchange membrane so as to imbed the electrode in the membrane, and is made gas- and liquid-permeable to permit the gas generated at the contact interface between the membrane and the electrode by the electrolysis to easily escape from the electrode. Such porous electrodes usually comprise a porous material prepared by uniformly mixing active particles functioning as an anode or cathode, a binder and, preferably a conductive material such as graphite or the like and forming the mixture into a thin film.
However, investigations by the inventors have revealed that, in the case of using an electrolytic cell wherein the above-described electrodes are directly bound to an ion-exchange membrane, the anode, for example, in the electrolytic cell comes into contact with hydroxide ion reversely diffusing from a cathode chamber, and is therefore required to possess alkali resistance as well as conventionally required chlorine resistance, thus a special and expensive material inevitably being selected for the electrode. In addition, though the life of the electrode is usually largely different from that of the ion-exchange membrane, both the electrode and the ion-exchange membrane bound to each other must be discarded when one of them has reached the end of its useful life. Therefore, where an expensive noble metal anode is used, there results a serious economic loss.
As a result of continuing studies on an electrolytic process imposing no such disadvantages and requiring as low a cell voltage as possible, the inventors have formerly discovered that alkali metal hydroxide and chlorine can be obtained, with substantially attaining the above-described object, by applying an unexpectedly low voltage when an alkali metal chloride aqueous solution is electrolyzed in an electrolytic cell wherein an anode or a cathode is disposed via a gas- and liquid-permeable porous layer with no electrode activity formed on the surface of a cation-exchange membrane, and have filed an application relating to this art as Japanese Patent Application No. 152416/79. Further investigations as to the disposition of electrodes have finally lead to the electrolyzing cell of the present invention for producing alkali metal hydroxide.